Complete Set of Polarization Transfer Observables for the 12C(p,n)^{12}{\rm C}(p,n) Reaction at 296 MeV and 0∘^{\circ}

A complete set of polarization transfer observables has been measured for the $^{12}{\rm C}(p,n)$ reaction at $T_p=296 {\rm MeV}$ and $\theta_{\rm lab}=0^{\circ}$. The total spin transfer $\Sigma(0^{\circ})$ and the observable $f_1$ deduced from the measured polarization transfer observables indicate that the spin--dipole resonance at $E_x \simeq 7 {\rm MeV}$ has greater $2^-$ strength than $1^-$ strength, which is consistent with recent experimental and theoretical studies. The results also indicate a predominance of the spin-flip and unnatural-parity transition strength in the continuum. The exchange tensor interaction at a large momentum transfer of $Q \simeq 3.6 {\rm fm}^{-1}$ is discussed.Comment: 4 pages, 4 figure

Differential cross section for neutron-proton bremsstrahlung

The neutron-proton bremsstrahlung process $(np \to np\gamma)$ is known to be sensitive to meson exchange currents in the nucleon-nucleon interaction. The triply differential cross section for this reaction has been measured for the first time at the Los Alamos Neutron Science Center, using an intense, pulsed beam of up to 700 MeV neutrons to bombard a liquid hydrogen target. Scattered neutrons were observed at six angles between 12$^\circ$ and 32$^\circ$, and the recoil protons were observed in coincidence at 12$^\circ$, 20$^\circ$, and 28$^\circ$ on the opposite side of the beam. Measurement of the neutron and proton energies at known angles allows full kinematic reconstruction of each event. The data are compared with predictions of two theoretical calculations, based on relativistic soft-photon and non-relativistic potential models.Comment: 5 pages, 3 figure

Two-step contribution to the spin-longitudinal and spin-transverse cross sections of the quasielastic (p,n) reactions

The two-step contribution to the spin-longitudinal and the spin-transverse cross sections of ^{12}C,^{40}Ca(p,n) reactions at 494 MeV and 346 MeV is calculated. We use a plane-wave approximation and evaluate the relative contributions from the one-step and the two-step processes. We found that the ratios of the two-step to the one-step processes are larger in the spin-transverse cross sections than in the spin-longitudinal ones. Combining these results with the distorted-wave impulse approximation (DWIA) results we obtained considerable two-step contributions to the spin-longitudinal and the spin-transverse cross sections. The two-step processes are important in accounting for the underestimation of the DWIA results for the spin-longitudinal and the spin-transverse cross sections.Comment: LaTeX 11 pages, 10 figure

Gamow-Teller strength distributions for nuclei in pre-supernova stellar cores

Electron-capture and $\beta$-decay of nuclei in the core of massive stars play an important role in the stages leading to a type II supernova explosion. Nuclei in the f-p shell are particularly important for these reactions in the post Silicon-burning stage of a presupernova star. In this paper, we characterise the energy distribution of the Gamow-Teller Giant Resonance (GTGR) for mid-fp-shell nuclei in terms of a few shape parameters, using data obtained from high energy, forward scattering (p,n) and (n,p) reactions. The energy of the GTGR centroid $E_{GT}$ is further generalised as function of nuclear properties like mass number, isospin and other shell model properties of the nucleus. Since a large fraction of the GT strength lies in the GTGR region, and the GTGR is accessible for weak transitions taking place at energies relevant to the cores of presupernova and collapsing stars, our results are relevant to the study of important $e^-$-capture and $\beta$-decay rates of arbitrary, neutron-rich, f-p shell nuclei in stellar cores. Using the observed GTGR and Isobaric Analog States (IAS) energy systematics we compare the coupling coefficients in the Bohr-Mottelson two particle interaction Hamiltonian for different regions of the Isotope Table.Comment: Revtex, 28 pages +7 figures (PostScript Figures, uuencoded, filename: Sutfigs.uu). If you have difficulty printing the figures, please contact [email protected]. Accepted for publication in Phys. Rev. C, Nov 01, 199

Roper excitation in p⃗+α→p⃗+α+X\vec{p}+\alpha \to \vec{p}+\alpha+X reactions

We calculate differential cross sections and the spin transfer coefficient $D_{nn}$ in the $\vec{p}+\alpha \to \vec{p}+\alpha+\pi^0$ reaction for proton bombarding energies from 1 to 10 GeV and $\pi^0 - p$ invariant masses spanning the region of the N$^*$(1440) Roper resonance. Two processes -- $\Delta$ excitation in the $\alpha$-particle and Roper excitation in the proton -- are included in an effective reaction model which was shown previously to reproduce existing inclusive spectra. The present calculations demonstrate that these two contributions can be clearly distinguished via $D_{nn}$, even under kinematic conditions where cross sections alone exhibit no clear peak structure due to the excitation of the Roper.Comment: 12 pages, 11 ps figures, Late

On the Strength of Spin-Isospin Transitions in A=28 Nuclei

The relations between the strengths of spin-isospin transition operators extracted from direct nuclear reactions, magnetic scattering of electrons and processes of semi-leptonic weak interactions are discussed.Comment: LaTeX, 8 pages, 1Postscript with figur

14Be(p,n)14B reaction at 69 MeV in inverse kinematics

A Gamow-Teller (GT) transition from the drip-line nucleus 14Be to 14B was studied via the (p,n) reaction in inverse kinematics using a secondary 14Be beam at 69 MeV/nucleon. The invariant mass method is employed to reconstruct the energy spectrum. A peak is observed at an excitation energy of 1.27(2) MeV in 14B, together with bumps at 2.08 and 4.06(5) MeV. The observed forward peaking of the state at 1.27 MeV and a good description for the differential cross section, obtained with a DWBA calculation provide support for the 1+ assignment to this state. By extrapolating the cross section to zero momentum transfer the GT-transition strength is deduced. The value is found to compare well with that reported in a beta-delayed neutron emission study.Comment: 5 pages, 2 figure

Determination of the Gamow-Teller Quenching Factor from Charge Exchange Reactions on 90Zr

Double differential cross sections between 0-12 degrees were measured for the 90Zr(n,p) reaction at 293 MeV over a wide excitation energy range of 0-70 MeV. A multipole decomposition technique was applied to the present data as well as the previously obtained 90Zr(p,n) data to extract the Gamow-Teller (GT) component from the continuum. The GT quenching factor Q was derived by using the obtained total GT strengths. The result is Q=0.88+/-0.06 not including an overall normalization uncertainty in the GT unit cross section of 16%.Comment: 11 papes, 4 figures, submitted to Physics Letters B (accepted), gzipped tar file, changed content

A Measurement of Gamow-Teller Strength for 176Yb -> 176Lu and the Efficiency of a Solar Neutrino Detector

We report a 0-degree 176Yb(p,n)176Lu measurement at IUCF where we used 120 and 160 MeV protons and the energy dependence method to determine GT matrix elements relative to the Fermi matrix element which can be calculated model independently. The data show that there is an isolated concentration of GT strength in the low lying 1+ states making the proposed Low Energy Neutrino Spectroscopy (LENS) detector (based on neutrino captures on 176Yb) sensitive to 7Be and pp neutrinos and a promising detector to resolve the solar neutrino problem.Comment: 11 pages, LATEX, 4 eps figure

Do Hadronic Charge Exchange Reactions Measure Electroweak L = 1 Strength?

An eikonal model has been used to assess the relationship between calculated strengths for first forbidden beta decay and calculated cross sections for (p,n) charge exchange reactions. It is found that these are proportional for strong transitions, suggesting that hadronic charge exchange reactions may be useful in determining the spin-dipole matrix elements for astrophysically interesting leptonic transitions.Comment: 14 pages, 5 figures, Submitted to Physical Review